Method of forming a photographic color image

ABSTRACT

PCT No. PCT/EP90/00726 Sec. 371 Date Apr. 7, 1992 Sec. 102(e) Date Apr. 7, 1992 PCT Filed Apr. 24, 1990 PCT Pub. No. WO90/13061 PCT Pub. Date Nov. 1, 1990.A method of forming a dye image in a photographic silver halide element containing a dye-providing compound and having in a layer thereof an imagewise distribution of catalytic silver which comprises the step of treating the material with a redox amplifying solution comprising a reducing agent and a redox amplification oxidant characterized in that the redox amplification oxidant is removed from the solution after use and the so-treated solution is re-used after the addition of fresh redox amplification oxidant.

This invention relates to a method of forming a photographic colourimage and in particular, to a method of forming such an image by a redoxamplification process.

Redox amplification processes have been described, for example inBritish Specification Nos. 1,268,126, 1,399,481, 1,403,418 and1,560,572. In such processes colour materials are developed to produce asilver image (which may contain only small amounts of silver) and thentreated with a redox amplifying solution to form a dye image. The redoxamplifying solution contains a reducing agent, for example a colourdeveloping agent, and an oxidising agent which is more powerful thansilver halide and which will oxidise the colour developing agent in thepresence of the silver image which acts as a catalyst. Oxidised colourdeveloper reacts with a colour coupler (usually contained in thephotographic material) to form image dye. The amount of dye formeddepends on the time of treatment or the availability of colour couplerrather than the amount of silver in the image as is the case inconventional colour development processes. Examples of suitableoxidising agents include peroxy compounds including hydrogen peroxide,cobalt (III) complexes including cobalt hexammine complexes, andperiodates. Mixtures of such compounds can also be used.

Since the amplifying solution contains both an oxidising agent and areducing agent it is inherently unstable. The best reproducibility forsuch a process is obtained by using a "one shot" system, where theoxidant is added to the developer and the solution mixed and usedimmediately (or after a short built in delay) and then discarded. Thisleads to the maximum solution usage possible with maximum effluent, anda processor has to be designed which uses small volumes of solution (amajor difficulty) in order to minimize the effluent. As a resultchemical costs are a maximum and the whole system is unattractiveespecially for a minilab environment where minimum effluent is required.It is believed that it is this that has inhibited commercial use of thisprocess.

The present invention provides a method by which amplification may beachieved while overcoming the disadvantages of unstable processingsolutions.

According to the present invention there is provided a method of forminga dye image in a photographic silver halide element containing adye-providing compound and having in a layer thereof an imagewisedistribution of catalytic silver which comprises the step of treatingthe material with a redox amplifying solution comprising a reducingagent and a redox amplification oxidant characterised in that the redoxamplification oxidant is removed from the solution after use and theso-treated solution is re-used after the addition of fresh redoxamplification oxidant.

FIG. 1 is a plot of the sensitometries of each of strips (1), (2), and(3), which are described in detail in Example 1, below.

FIG. 2 is a plot of the sensitometries of strips (4) and (5), alsodescribed in detail in Example 1, below.

FIG. 3 is a plot comparing the sensitometries of strips (2), (3), and(5), which are described in detail in Example 1, below.

FIG. 4 is a plot of the sensitometries of strips (6), (7), and (8),which are described in detail in Example b 2, below.

FIG. 5 is a plot of the sensitometries of strips (9) and (10), which aredescribed in detail in Example 2, below.

FIG. 6 is a plot comparing the sensitometries of strips (6), (8), and(8), which are described in detail in Example 2, below.

FIG. 7 is a schematic of a redox amplification system in accordance withthe invention. The system is described in detail in several laterparagraphs.

FIG. 8 is a schematic of an electrolytic cell for removal of hydrogenperoxide from the developer-amplifier solution, as described below inExample 3.

FIG. 9 is a plot of hydrogen peroxide concentration against time forseveral metal and metal oxide catalysts examined for peroxide removal,as described below in Example 5.

FIG. 10 is a plot of sulphate, sulphite, and CD3 versus ion exchangecolumn bed-volume, as described in detail in Example 6, below.

The dye-providing compound may be, for example, a dye developer, a redoxdye releaser or a coupler capable of reacting with oxidised colourdeveloper to form an image dye with or without the concommitant releaseof a photographically useful group. They may be incorporated into thephotographic material by known means.

The reducing agent may, for example, be a colour developing agent, ablack-and-white developing agent (or electron transfer agent) or animage modifier, interlayer scavenger, preservative or stain reducer,e.g. a sulphite, hydroxylamine or a substituted hydroxylamine. If anelectron transfer agent is used, its oxidised form may be employed tooxidise a redox dye releaser which, in turn, will release a dye. If thereducing agent is a sulphite or hydroxylamine it will be present for anyof the reasons noted above but will not take part in the image-formingprocess; its presence will, however, modify the stability of thesolution. In such a system the reducing agent involved in the colourforming reaction need not be in the redox amplification solution butcould be incorporated in the photographic material or applied from aseparate bath.

In a preferred form of the present invention the reducing agent is acolour developing agent. Preferred colour developing agents arephenylene diamines. Especially preferred developing agents are4-amino-3-methyl-N,N-diethylaniline hydrochloride,4-amino-3-methyl-N-ethyl-N-β(methanesulphonamido)ethylaniline sulphatehydrate,4-amino-3-methyl-N-ethyl-N-β(methanesulphonamido)ethyl-N,N-diethylanilinehydrochloride and 4-amino-N-ethyl-N-(2-methoxyethyl)-m-toluidinedi-p-toluene sulphonate.

The invention has the following advantages:

(a) The treated used solution is stable and contains a known amount ofoxidant (ideally zero). Consequently it can be kept for long periods inthe stable condition and then reused by adding the required amount ofoxidant and replenishing in the normal way (i.e. taking account of theamount of material that has been processed through the solution).

(b) Processor design would become simpler because larger volumes ofsolution could be used, kept and regenerated compared to a processordesigned for one-shot operation.

(c) Effluent volume could be reduced to low levels.

The redox amplification solution will, in a preferred embodiment containa colour developing agent and the silver halide material will contain acolour coupler. In an alternative embodiment the redox amplificationsolution will contain an electron transfer agent as reducing agent andthe silver halide material will contain a redox dye-releasing compound.

The redox amplification oxidant maybe a peroxide, a cobalt (III) complexor a periodate, and is preferably hydrogen peroxide the source of whichmay be an aqueous solution of hydrogen peroxide or a compound capable ofreleasing it. The following description concerns hydrogen peroxide, butit is believed that methods of removing other oxidants could be devised.

A number of ways of removing hydrogen peroxide from amplifier solutionswill now be described.

(1) Electrolytic reduction at a cathode:

    2H.sup.+ +2e.sup.- +H.sub.2 O.sub.2 →2 H.sub.2 O

with or without the addition of extra sulphite for added protection. Oneadvantage is that non-oxidised developer is unaffected and oxidiseddeveloper may even be reduced back to its non-oxidised form at thecathode. The type of cathode may be very important and an acceptableanodic reaction would have to be chosen or the anode would have to beseparated via a semi-permeable or anionic membrane. Migration of the HO₂⁻ ion from the cathode would also help. Preferred electrode materialsare titanium, platinum, platinum-rhodium, platinum coated titanium andsilver. The electrodes may be rough or smooth and may be coated withmanganese dioxide.

(2) Certain compounds (scavengers) may be preferentially oxidised(rather than colour developer) by H₂ O₂ and so could be usedsacrificially to remove the peroxide. A redox indicator dye may serve toshow when enough reducing compound has been added. Examples of suchcompounds are hydroquinones, ballasted hydroquinones, hydrazines,aldehydes and compounds capable of tautomerising to give an enediolform, for example, ascorbic acid, reductone, methyl reductinic acid,dihydroxy acetone, 2,4-dihydroxy-4-methyl-1-piperidinocyclopenten-3-one(piperidino hexose reductone), catechol, ascorbyl palmitate andchromanols. Inorganic scavengers may be dithionites or phosphites. Aparticularly useful class of inorganic scavengers comprises watersoluble or water insoluble sulphites and metabisulphites, e.g. sodiummetabisulphite. Such scavengers may be added as solids or solutions andhave the advantages of speed, inexpensiveness and do not cause loss ofcolour developing agent. Alternatively the scavenger could be coated ina layer of the photographic material being processed, e.g. as a toplayer on the back of the material.

(3) Mordanted oxidisable dye. If this were in a cartridge with a windowsome indication of the state of the cartridge could be obtained and soit could be replaced when necessary.

(4) Catalytic decomposition and oxygen removal. Catalysts are numerous,the main criterion being small particle size, for example Mn, Ni, Pt,Ag, Pd Glass, Fe, manganous salts, manganous hydroxide, MnO₂, compoundswhich provide manganous hydroxide or MnO₂, catalase, black magnetic ironoxide (Fe₃ O₄), ferrous salts, black copper oxide and cupric salts. Itwould be most advantageous if the catalytic surface could aid in"fixing" the oxygen e.g. SO₃ ₌ +O→SO₄ ⁼ ; sulphite being supplied fromsolution. Alternatively metal+O→metal oxide. The catalytic activity maybe regenerated electrolytically by cathodic reduction. The preferredmethods use manganese dioxide, catalase, palladium black, Adams platinumoxide catalyst, ground pumice and cathodic electrolysis. Alternativelythe catalyst could be coated in a layer of the photographic materialbeing processed, e.g. as a top layer on the back of the material.

(5) Combined oxygen permeable membrane and catalyst. Decompose theperoxide at the membrane surface and allow oxygen to diffuse into an airspace (c.f. removal of NH₃ from developers with yeast bags).

(6) Vacuum should favour decomposition because of the formation of a gasi.e. by subjecting a thin film of the solution to a vacuum it may bepossible to pull off the oxygen from a catalytic surface.

(7) Decomposition of hydrogen peroxide in the presence of a catalyst isaccelerated by ultrasonic agitation. Cavitation may favour suchdecomposition.

(8) Dialysis, through semipermeable membranes, of the used solution toremove the hydrogen peroxide using a closed loop for the extractionsolution. The process should be arranged so that the maximumconcentration difference in hydrogen peroxide will exist across themembrane and depends on the non-passage (or reduced passage) of reducingagent through the membrane. Any chloride ion released in theamplification process would also be extracted and this could be an addedadvantage.

(9) The used amplification solution is iled under reduced pressure. Thevapour in equilibrium with the solution will be a mixture of H₂ O and H₂O₂. If this vapour is drawn off and passed over a catalyst the hydrogenperoxide may then be decomposed to oxygen and water. The water could becondensed and returned to the main solution and the oxygen would beexhausted and discarded via the vacuum pump.

The colour photographic material to be processed may be of any type butwill preferably contain low amounts of silver halide. Preferred silverhalide coverages are in the range 10-200 mg/m² (as silver). The materialmay comprise the emulsions, sensitisers, couplers, supports, layers,additives, etc. described in Research Disclosure, December 1978, Item17643, published by Kenneth Mason Publications Ltd, Dudley Annex, 12aNorth Street, Emsworth, Hants P010 7DQ, U.K.

In a preferred embodiment the photographic material comprises aresin-coated paper support and the emulsion layers comprise more than80%, preferably more than 90% silver chloride and are more preferablycomposed of substantially pure silver chloride. Preferably theamplification solution contains hydrogen peroxide and a colourdeveloping agent.

The photographic materials can be single colour materials or multicolourmaterials. Multicolour materials contain dye image-forming unitssensitive to each of the three primary regions of the spectrum. Eachunit can be comprised of a single emulsion layer or of multiple emulsionlayers sensitive to a given region of the spectrum. The layers of thematerials, including the layers of the image-forming units, can bearranged in various orders as known in the art.

A typical multicolour photographic material comprises a support bearinga yellow dye image-forming unit comprised of at least one blue-sensitivesilver halide emulsion layer having associated therewith at least oneyellow dye-forming coupler, and magenta and cyan dye image-forming unitscomprising at least one green- or red-sensitive silver halide emulsionlayer having associated therewith at least one magenta or cyandye-forming coupler respectively. The material can contain additionallayers, such as filter layers.

Normally in the commercial environment, processing of photographicmaterials is performed by machine and increasingly by small machines ofthe minilab type. In the present case it is desirable for the tank inwhich redox amplification takes place to have a small as possible volumeto minimise chemical costs. The oxidant removal may be performedcontinuously or only when the machine is idle. It might, for example, bedesirable to initiate oxidant removal only if the machine has been idlefor ten minutes.

In either case it may be that the destruction of oxidant also removesother wanted components such as colour developer, sulphite ions orhydroxylamine compounds. To compensate for this, these components inaddition to the normal replenisher will need to be added to theamplification bath.

A particular arrangement is illustrated in FIG. 7 of the accompanyingdrawings in which there is schematically shown the amplification tank(1) of a processing machine provided with material drive rollers (2),inlet (3) and outlet means (5) for the material to be processed. Themachine has inlet means for the replenishment of processing solution (6)and overflow means (7). Associated with the tank are pumps (8), pipes(9), peroxide removal cartridge (10), aqueous hydrogen peroxide tank(11), replenisher tank (12), additional replenisher tank (13) and mixer(14) and anion exchange resin cartridge (15) to remove unwanted chlorideand bromide ions. Instead of the peroxide removal cartridge the solutioncould be fed to a tank where the removal takes place. Locating the anionexchange cartridge after peroxide removal ensures that there is nointeraction with hydrogen peroxide, as it has been removed.Alternatively the anion exchange cartridge could be located beforeperoxide removal. In such a case removal of chloride and bromide ionswould be advantageous if silver was used as the peroxide decompositioncatalyst.

In use, the amplification solution (16) may be pumped to the treatmentand replenishment stations continuously or intermittently as desired,e.g. when the machine has been idle for a specified time period. Theamplifier solution is pumped to the cartridge (10) containing, forexample, a catalyst for the decomposition of hydrogen peroxide. Thetreated solution is then fed to the replenisher tank (12). In one modeof operation this would only happen when the machine was idle and inthis case the supply of oxidant and replenisher to the mixer (14) wouldbe shut off. During operation the amplifier solution would bereplenished by feeding the required amounts of oxidant and replenisherto the tank via the mixer. Alternatively, for continuous oxidantdestruction a regime of oxidant destruction and replishment would beestablished.

If oxidant destruction is also causing some loss of other oxidisablecomponents, their loss could be compensated for by feeding in a secondreplenisher from tank (13).

In an alternative method the cartridge (10) could be dispensed with bycirculating through tank (1) a coated material containing, say, acatalyst for the decomposition of hydrogen peroxide. The peroxidedestruction would then take place inside the tank itself. Recirculationand replenishment could be achieved as described above except that thecartridge (10) would be absent.

The following examples are included for a better understanding of theinvention.

In the examples below, in the case of the removal of hydrogen peroxideby catalysts, the developer/amplifier solution containing the hydrogenperoxide was mixed with a small amount of the catalyst in a roundbottomed flask while nitrogen was passed through the solution and thepressure reduced in order to sweep away any oxygen formed. It is notknown whether these attempts to remove oxygen by sweeping the solutionwith nitrogen under reduced pressure are important. The length oftreatment and the amount of catalyst required have also not beeninvestigated.

In the case of electrolysis a simple H cell was used with a remote anodesuch that cathode and anode compartments could be isolated from eachother with a tap and the catholyte examined.

The success of the method of removal can be judged by running anamplification process using the solution in question. If the peroxidehas been removed no amplification will occur. An additional check canalso be made by testing the solution with lead sulphide test paper whena white spot indicates the presence of peroxide.

EXAMPLE 1

The following solutions were made up:

    ______________________________________                                        Developer/amplifier Solution A                                                Na.sub.2 SO.sub.3       1.0     g                                             Na.sub.2 CO.sub.3       20.0    g                                             CD3                     5.0     g                                             KBr                     0.001   g                                             Na.sub.2 EDTA.2H.sub.2 O                                                                              0.1     g                                             Distilled Water to      960     ml                                            Hydrogen Peroxide Solution B                                                  Hydrogen Peroxide 100 VOL                                                                             8.0     ml                                            Distilled Water to      20.0    ml                                            Antifoggant Solution C                                                        Acetamido PMT           0.0436  g                                             Na.sub.2 CO.sub.3       0.010   g                                             Distilled Water to      40.0    ml                                            Antistain Solution D                                                          Ascorbic Acid           0.436   g                                             Distilled Water to      40.0    ml                                            ______________________________________                                    

MULTILAYER COATING

A colour paper of similar construction to currently available silverchloride paper was prepared. All the emulsions were substantially puresilver chloride and the silver coverage in the three image layers were:yellow 49.5 mg/m² magenta 21.5 mg/m², cyan 19.4 mg/m² giving a totalsilver coverage of 90.4 mg/m².

The following strips were processed

STRIP 1--fresh solution, no H₂ O₂ present (control)

A fresh solution was prepared of 96 ml of the developer/amplifiersolution A, 1.0 ml antifoggant solution C, and 0.25 ml antistainsolution D. An exposed strip (1) of the multilayer coating with reducedsilver coverage (exposed to a four colour wedge to give Cyan, Magenta,Yellow and neutral wedges) was then processed in the solution for 60secs at 350° C. The full process is indicated below. Low densities wereobserved due to normal colour development without redox amplification.The sensitometry for strip (1) is shown in FIG. 1.

    ______________________________________                                        PROCESS                                                                       ______________________________________                                        Develop/amplify  60 secs                                                      Stop 2% acetic acid                                                                            30 secs                                                      Wash             30 secs                                                      RA4 Bleach Fix   30 secs                                                      Wash             120 secs                                                     Temperature      35° C.                                                Processing method                                                                              H11 DRUM                                                     ______________________________________                                    

STRIP 2--fresh solution, H₂ O₂ present (control) developer/amplifiersolution A and 0.5 ml peroxide resolution B. After adding 1.0 mlantifoggant solution C and 0.25 ml antistain solution D exposed strip(2) was immediately processed as described above. Normal redoxamplification observed. The sensitometry is shown in FIG. 1.

STRIP 3--Treated solution kept 1 hr with H₂ O₂ present

In a 500 ml three necked RB flask fitted with a nitrogen bubbler and awater condenser was placed a premix of 96 ml of developer/amplifiersolution A and 0.5 ml of peroxide solution B. A gradual flow of nitrogenwas bubbled through the solution while it was pumped to a pressure ofapproximately 35 mm Hg and the temperature was maintained at 35° C. for10 mins. The solution was removed, made up to volume and kept a further50 mins at 18° C. 1.0 ml antifoggant solution C and 0.25 ml antistainsolution D were added to the solution. An exposed strip (3) of themultilayer coating was processed in the solution for 60 secs at 35° C.as above. The sensitometry is shown in FIG. 1.

The comparison of the sensitometry of strips 1, 2 and 3 shown in FIG. 1represents the effect of (a) having no H₂ O₂ present and (b) the effectof keeping the peroxide solution (treated in the manner described) for 1hr. In the latter case a loss of shoulder contrast together with a lossof speed was observed (cf the fresh peroxide control).

STRIP 4--solution containing H₂ O₂ treated with MnO₂

The solution and method used to prepare strip 3 was repeated only thistime 0.15 g of black manganese dioxide was introduced into the flaskbefore the mixture of developer/amplifier (96 ml solution A) andperoxide (0.5 ml solution B) was added. The solution was pumped undernitrogen for 10 mins at 35° C., filtered and made up to volume withwater. 1.0 ml of the antifoggant solution C and 0.25 ml of the antistainsolution D was added and a strip (4) was processed as above. Very lowdensities were obtained due to the removal of the peroxide. Thesensitometry is shown in FIG. 2. Testing the solution with lead sulphidetest paper indicated that only a very low level of hydrogen peroxide waspresent.

STRIP 5--addition of H₂ O₂ as solution used for STRIP 4

While the solution was still on the processing drum 0.5 ml of theperoxide solution B was added and allowed to mix. A further strip (5)was then processed. The high densities were restored due to redoxamplification. A comparison of the sensitometry of strips 4 and 5 isshown in FIG. 2.

The results shown in FIGS. 1 and 2 indicate that hydrogen peroxide canbe successfully removed from a working developer/amplifier solution.FIG. 3 shows a comparison of strip 2 (control with peroxide present), 3(control kept 1 hr H₂ O₂ present) and 5 (peroxide removed with MnO₂ andthen H₂ O₂ readded).

EXAMPLE 2 (strips 6-10)--The long term stability of solutions which havehad peroxide removed

STRIP 6--Fresh control

A fresh control (strip 6) was repeated as described under Strip 2. Highdensities were observed showing the result of a normal Redox process.The sensitometry is shown in FIG. 4.

STRIP 7 effect of keeping the control solution 46 hrs with H₂ O₂ 0present

The method described under STRIP 3 was repeated only with the followingdifferences:

a mixture of 192 ml developer/amplifier solution A was taken with 1.0 mlperoxide solution B. A gradual flow of nitrogen was bubbled through thesolution while it it was pumped and the temperature was maintained at18° C. for 20 mins. 96 ml of this solution was then placed in a 500 mlstoppered conical flask under nitrogen and left in a water bath at 35°C. for 46 hrs. During this time the solution had turned dark brown andformed a black precipitate. The solution was filtered, made up tovolume. 1.0 ml antifoggant solution C, 0.25 ml antistain solution D wasadded and a strip (7) was processed as described above. This time thedensities observed were low indicating the loss of peroxide and ageneral deterioration of the solution. The sensitometry is shown in FIG.4.

STRIP 8--addition of H₂ O₂ to solution used for strip 7.

While the solution used to process strip 7 was still on the drumprocessor 0.5 ml peroxide solution was added, allowed to mix and anotherstrip (8) was processed. Appreciably higher densities were observedindicating that on leaving a developer/amplifier solution containingperoxide for 46 hrs at 35° C. the peroxide disappears of its own accordand that some photographic activity can then be restored by there-addition of more peroxide. The sensitometry for strip 8 is shown inFIG. 4.

STRIP 9--Peroxide removal with MnO₂, and solution left 46 hrs

The example shown under strip 7 was repeated only this time 0.3 g ofblack manganese dioxide was added to the flask at the start of theexperiment. As before the solution was bubbled with nitrogen and pumpedfor 20 mins at 18° C. The solution was filtered and left under nitrogenfor 46 hrs at 35° C. After this time the solution was a light browncolour and did not contain a precipitate. After the addition of theantifoggant and antistain solutions, a strip (9) was processed. Lowdensities were observed indicating the removal of the peroxide. Thesensitometry is shown in FIG. 5.

While the solution used to process strip 9 was still on the drumprocessor 0.5 mi peroxide solution was added, allowed to mix and anotherstrip was processed. High densities were observed (higher than for strip8). The sensitometry for strip 10 is shown in FIG. 5.

FIG. 6 shows a comparison of (a) a fresh redox process, (Strip 6), (b)the result when a redox solution containing peroxide is kept 46 hrs at35° C. and is then rejuvenated by the addition by the addition of moreperoxide, (Strip 8), and (c) the result when a solution containingperoxide is treated initially to remove the peroxide, kept 46 hrs at 35°C. and then rejuvenated by adding more peroxide (Strip 10). A higherdensity is observed for (c) indicating the advantage of removing theperoxide rapidly.

EXAMPLE 3

Removal of hydrogen peroxide from the developer-amplifier solution byelectrolytic reduction at a cathode

The electrolytic cell shown in FIG. 8 was constructed. The three anodesand two cathodes were made of perforated stainless steel and wereapprox. 10 cm by 10 cm. The electrodes were separated by sheets ofsemipermeable membrane (Gallenkamp PJC-400-070F, Visking, size 5-24/32)and the average electrode separation was 3.0 mm. The 3 anodes wereconnected together and likewise the 2 cathodes were also connectedtogether. A recirculation system was arranged in the cathodecompartments so that a larger quantity (1 liter) of developer than thecell capacity (250 ml) could be treated. The anolyte of the cell was asolution of sodium bicarbonate (16 g/l ) and was not recirculated. Thefollowing developer-amplifier solution was made up:

    ______________________________________                                        Sodium sulphite     1.91     g                                                Sodium carbonate    14.6     g                                                CD3                 5.24     g                                                1-hydroxyethylidene-1,1-                                                                          0.825    g                                                diphosphonic acid                                                             Diethyl hydroxylamine                                                                             0.752    g                                                Antifoggant Soln C (Example 1)                                                                    3.04     ml                                               Potassium Chloride  0.117    g                                                Potassium Bromide   0.001    g                                                2N sulphuric acid   16.7     ml                                               Distilled water     840      ml                                               Sodium hydroxide    1.58     g                                                Final volume to     1000     ml pH 10.2                                       ______________________________________                                    

This solution gave the sensitometric parameters shown in Table 1(below), strip 11 showing the result without amplification. Whenhydrogen peroxide was added at the rate of 1.29 ml (solution B,Example 1) per 100 ml developer-amplifier solution the amplified imageparameters represented by strip 12 were obtained.

To the bulk (1000 ml) of the developer-amplifier solution, 12.9 mlhydrogen peroxide solution was added and the mixture was placed in thereservoir and pumped through the cell. A current of 4 amps (currentdensity 12.5 ma/sq cm) was maintained for 90 mins. During this time thepH increased (pH 12).

Analysis of the solution showed that the hydrogen peroxide had beenremoved together with some of the following components--diethylhydroxylamine, sulphite and some colour developing agent. The followingadditions were made (1 liter developer-amplifier) to compensate for thesensitometric effects of these losses:--Diethyl hydroxylamine 0.37 g,sodium bicarbonate 9.4 g, sodium sulphite 1.53 g, and CD3 1.0 g(representing approx. a 19% loss probably via the membrane). A portionof this readjusted solution was used to process a strip 13 directly andshowed no amplification due to the successful removal of the hydrogenperoxide.

On the readdition of the peroxide at the rate indicated above thesensitometric parameters shown by strip 14 were obtained indicating thatthe amplified sensitometry had been essentially restored (c.f. strips 12and 14 Table 1).

                                      TABLE 1                                     __________________________________________________________________________             Inertial Speed                                                                          Dmin      Dmax      Contrast                               Strip Number                                                                           Red                                                                              Green                                                                             Blue                                                                             Red                                                                              Green                                                                             Blue                                                                             Red                                                                              Green                                                                             Blue                                                                             Red                                                                              Green                                                                             Blue                            __________________________________________________________________________    11                                                                              (no H.sub.2 O.sub.2)                                                                 121                                                                              113 116                                                                              0.096                                                                            0.118                                                                             0.099                                                                            0.640                                                                            0.730                                                                             0.736                                                                            0.827                                                                            0.932                                                                             1.028                           12                                                                              (+ H.sub.2 O.sub.2)                                                                  121                                                                              112 121                                                                              0.122                                                                            0.132                                                                             0.110                                                                            2.636                                                                            2.650                                                                             2.337                                                                            3.673                                                                            4.426                                                                             3.530                           13                                                                              electrolysed                                                                         112                                                                              107 109                                                                              0.096                                                                            0.119                                                                             0.097                                                                            0.621                                                                            0.696                                                                             0.778                                                                            0.801                                                                            0.848                                                                             1.046                             (no H.sub.2 O.sub.2)                                                          (+ additives)                                                               14                                                                              electrolysed                                                                         117                                                                              109 117                                                                              0.126                                                                            0.136                                                                             0.113                                                                            2.711                                                                            2.687                                                                             2.445                                                                            3.996                                                                            4.796                                                                             3.857                             (+ H.sub.2 O.sub.2)                                                           (+ additives)                                                               __________________________________________________________________________

EXAMPLE 4

The removal of hydrogen peroxide from a developer-amplifier solutionusing a scavenger

To 100 ml of the developer-amplifier solution of Example 3 was added1.29 ml of the hydrogen peroxide solution B, followed by the scavenger0.25 g of 2,4-dihydroxy-4-methyl-1-piperidinocyclopenten-3-one(sometimes known as piperidino hexose reductone and referred to below asPHR). The solution was shaken to dissolve the compound and then leftapprox. 60 mins and analysed for H₂ O₂. A sample of thedeveloper-amplifier solution (without the PHR) was also analysed (1)directly after the addition of the peroxide and also (2) after 60 mins.The results are shown in Table 2. A high proportion of the hydrogenperoxide had been removed by the scavenger.

                  TABLE 2                                                         ______________________________________                                        Solution          Age       g/l H.sub.2 O.sub.2                               ______________________________________                                        Dev-amp + H.sub.2 O.sub.2                                                                       Fresh (5')                                                                              1.53                                              Dev-amp + H.sub.2 O.sub.2                                                                       (60')     1.38                                              Dev-amp + H.sub.2 O.sub.2 + PHR                                                                 (60')     0.22                                              ______________________________________                                    

The effect on the photographic performance of adding the scavenger isshown in Table 3 (below). The sensitometric parameters obtained forstrip 15 are without the addition of H₂ O₂ to the developer-amplifier(i.e. showing no amplification). Strip 16 is the result obtained on theaddition of the hydrogen peroxide (i.e. showing normal amplification ).Strip 17 shows the effect on the sensitometry after adding 0.25 g PHRand leaving the solution (occasional shaking) for about 60 mins. beforeprocessing. A considerable reduction in amplification is observed due tothe scavenging of the hydrogen peroxide. Readdition of hydrogen peroxideto this solution (strip 18) shows that amplification can bereestablished and the result may be compared with strip 16 parameters.

                                      TABLE 3                                     __________________________________________________________________________           Inertial Speed                                                                          Dmin      Dmax      Contrast                                 Strip Number                                                                         Red                                                                              Green                                                                             Blue                                                                             Red                                                                              Green                                                                             Blue                                                                             Red                                                                              Green                                                                             Blue                                                                             Red                                                                              Green                                                                             Blue                              __________________________________________________________________________    15 (no H.sub.2 O.sub.2)                                                              137                                                                              133 134                                                                              0.094                                                                            0.115                                                                             0.072                                                                            0.500                                                                            0.501                                                                             0.607                                                                            0.532                                                                            0.524                                                                             0.773                             16 (+ H.sub.2 O.sub.2)                                                               137                                                                              131 135                                                                              0.111                                                                            0.135                                                                             0.086                                                                            2.552                                                                            2.613                                                                             2.674                                                                            2.906                                                                            3.142                                                                             3.722                             17 (no H.sub.2 O.sub.2)                                                              138                                                                              132 134                                                                              0.097                                                                            0.120                                                                             0.076                                                                            0.844                                                                            1.168                                                                             1.796                                                                            0.904                                                                            1.356                                                                             2.099                               (+ PHR)                                                                     18 (+ H.sub.2 O.sub.2)                                                               139                                                                              132 143                                                                              0.109                                                                            0.131                                                                             0.085                                                                            2.501                                                                            2.693                                                                             2.687                                                                            2.853                                                                            3.575                                                                             3.519                             __________________________________________________________________________

EXAMPLE 5

A developer-amplifier of the composition shown below was made up and aninitial sample taken for analysis. Solid sodium metabisulphite (5 g/l)was added with vigorous stirring. Samples were taken and analysed byiodine/thiosulphate titration for hydrogen peroxide content.

    ______________________________________                                        Dev-amp composition                                                           ______________________________________                                        Potassium carbonate                                                                              20.0       g/l                                             EDTA (Na.sub.2)    0.1        g/l                                             Diethylhydroxylamine                                                                             2.0        ml/l                                            CD3                8.0        g/l                                             Hydrogen peroxide (30%)                                                                          5.0        ml/l                                            ______________________________________                                    

The hydrogen peroxide level after the addition of sodium metabisulphitewas found to be as follows:

    ______________________________________                                        TIME (min)    H.sub.2 O.sub.2 (ml/l, 30%)                                     ______________________________________                                        0             4.7                                                             0.5           1.9                                                             1.0           0.8                                                             2.0           0.0                                                             ______________________________________                                    

After two minutes all the hydrogen peroxide had reacted. The absence ofhydrogen peroxide was confirmed in a separate experiment by the additionof manganese dioxide before and after sulphite addition. Manganesedioxide decomposes hydrogen peroxide with the visible evolution ofoxygen. This ceased completely 2 minutes after adding metabisulphite.

5 ml/l of 30% hydrogen peroxide is equivalent to 0.044 M and 5 g/l ofsodium metabisulphite is equivalent to 0.056 M sulphite ion. A slightexcess of total sulphite ion appears to be necessary for completeremoval of peroxide.

As a comparison several metal and metal oxide catalysts (at 0.5 g/l)were examined for peroxide removal and a plot of hydrogen peroxideagainst time for these is shown in FIG. 9, with metabilsulphite on thesame plot.

EXAMPLE 6

A variant of Example 5 was performed in which 6 g/l sodiummetal)isulphite was added to the dev-amp solution and after 5 minutesthe solution was passed through an ion-echange column (IRA 400) in orderto remove excess sulphite and sulphate. It can be seen in FIG. 10 thatboth sulphite and sulphate are completely removed in the first bedcolume but only partially removed subsequently. CD3 was lowered slightlyin the first bed volume but then remained constant.

Another version of this procedure was carried out in which theion-exchange column was regenerated using potassium sulphite and thenwashed. The dev-amp solution was then passed through the column toeffect peroxide removal. Peroxide was effectively removed on passagethrough the column lbut sulphite was displaced from the column and wentinto solution.

EXAMPLE 7

Bisulphite ion forms an addition complex with formaldehyde and otheraldehydes and these are used in some black and white developer solutionsas a controlled source of low levels of sulphite. The addition of sodiumformaldehyde-bisulphite (1 g/l) to the dev-amp did remove peroxide butat a fairly slow rate as shown in the table below.

    ______________________________________                                        TIME (min)    H.sub.2 O.sub.2 (ml/l, 30%)                                     ______________________________________                                        0             5.10                                                            2.5           4.40                                                            5.5           4.09                                                            10.25         3.95                                                            15.25         3.78                                                            20            3.55                                                            25            3.43                                                            30            3.30                                                            ______________________________________                                    

The amount of sodium formaldehyde bisulphite at 1 g/l is probably toolow to completely remove the hydrogen peroxide although it was observedthat formaldehyde itself also reacts with peroxide which provides someadditional capacity.

EXAMPLE 8

Sodium glutaraldehyde bisulphite reacts much faster than theformaldehyde compound as shown in the table below.

    ______________________________________                                        TIME (min)    H.sub.2 O.sub.2 (ml/l, 30%)                                     ______________________________________                                        0             4.97                                                            2             1.93                                                            5             0.45                                                            10            0.22                                                            ______________________________________                                    

The dev-amp solution became very dark during this time as if the colourdeveloping agent was being oxidised. The solution, however, was notanalysed for CD3 content which might have confirmed this.

We claim:
 1. A method of forming a dye image in a photographic silverhalide element containing a dye-providing compound and having in a layerthereof an imagewise distribution of catalytic silver which comprisesthe step of treating the material with a redox amplifying solutioncomprising a reducing agent and a redox amplification oxidantcharacterised in that the redox amplification oxidant is removed fromthe solution after use and the so-treated solution is re-used after theaddition of fresh redox amplification oxidant.
 2. A method as claimed inclaim 1 in which the dye-providing compound is a colour coupler.
 3. Amethod as claimed in claim 1 in which the reducing agent is a colourdeveloping agent.
 4. A method as claimed in any of claim 1 in which theoxidant is hydrogen peroxide.
 5. A method as claimed in claim 1 in whichthe oxidant is removed continuously or intermittently.
 6. A method asclaimed in claim 5 in which the oxidant is removed when processing isnot taking place.
 7. A method as claimed claim 4 in which the hydrogenperoxide is removed by decomposing it by contact with a catalyst for thedecomposition of hydrogen peroxide.
 8. A method as claimed in claim 4 inwhich the hydrogen peroxide is removed by dialysis through asemipermeable membrane.
 9. A method as claimed in claim 4 in which thehydrogen peroxide is removed by treatment with a hydrogen peroxidescavenger.
 10. A method as claimed in claim 9 in which the hydrogenperoxide scavenger is a water soluble sulphite or metabisulphite.
 11. Amethod as claimed in claim 4 in which the hydrogen peroxide is removedby electrolysis with or without a semipermeable or anionic membrane. 12.A method as claimed in claim 1 in which the material is a multicolourphotographic material comprising a support bearing a yellow dyeimage-forming unit comprised of at least one blue-sensitive silverhalide emulsion layer having associated therewith at least one yellowdye-forming coupler, and magenta and cyan dye image-forming unitscomprising at least one green- or red-sensitive silver halide emulsionlayer having associated therewith at least one magenta or cyandye-forming coupler respectively.